Method for the production of unsaturated nitriles from alkanes

ABSTRACT

The invention relates to a process for preparing unsaturated nitrites from the corresponding alkanes which comprises the steps:
         a) feeding an alkane into a dehydrogenation zone and catalytically dehydrogenating the alkane to the corresponding alkene to obtain a product gas stream A which comprises the alkene, unconverted alkane and possibly one or more further gas components selected from the group consisting of steam, hydrogen, carbon oxides, hydrocarbons having a lower boiling point than the alkane or the alkene (low-boilers), nitrogen and noble gases,   b) at least partially removing further gas components from the product gas stream A to give a feed gas stream B which comprises the alkane and the alkene,   c) feeding the feed gas stream B, ammonia, an oxygen-containing gas and, if desired, steam into an oxidation zone and catalytically ammoxidizing the alkene to the corresponding unsaturated nitrile to give a product gas stream C which comprises the unsaturated nitrile, ammoxidation by-products, unconverted alkane and alkene and possibly one or more further gas components selected from the group consisting of steam, oxygen, carbon oxides, ammonia, nitrogen and noble gases,   d) optionally removing ammonia from the product gas stream C to give an ammonia-depleted product gas stream D,   e) removing the unsaturated nitrile and ammoxidation by-products from the product gas stream C or D by absorption in an aqueous absorbent to give a gas stream E which comprises unconverted alkane and alkene and possibly one or more further gas components selected from the group consisting of oxygen, carbon oxides, ammonia, nitrogen and noble gases, and an aqueous stream which comprises the nitrile and the by-products, and recovery of the unsaturated nitrile from the aqueous stream,   f) recycling the gas stream E into the dehydrogenation zone.

The invention relates to a process for preparing unsaturated nitritesfrom alkanes.

It is known that unsaturated nitrites such as acrylonitrile andmethacrylonitrile can be prepared from the corresponding alkenes,propene and isobutene respectively, by what is known as ammoxidation ofthe alkene using an ammonia/oxygen mixture in the presence of a suitablecatalyst. The relevant alkenes may be prepared in a precedingdehydrogenation step from the corresponding alkanes.

For instance, ammoxidation of propene gives acrylonitrile andammoxidation of isobutene gives methacrylonitrile. In general, amethyl-substituted olefin yields the corresponding α,β-unsaturatednitrile while the methyl group is converted into a nitrile group.

EP-A 0 193 310 describes a process for preparing acrylonitrile frompropane which comprises catalytically dehydrogenating propane to givepropene, ammoxidizing propene to give acrylonitrile, removingacrylonitrile from the product gas stream of ammoxidation and recyclingunconverted propane and propene into the catalytic dehydrogenation.After removing acrylonitrile from the product gas stream of theammoxidation, the hydrogen formed in the dehydrogenation is selectivelycombusted using oxygen over an oxidation catalyst to give water, whichleaves a hydrogen-depleted gas stream comprising unconverted propane,propene, carbon oxides and low-boiling hydrocarbons. After removal of asub-stream and recovery of unconverted propane and propene therefrom,this gas stream is recycled into the dehydrogenation step.

A disadvantage of this process is that the hydrogen resulting from thedehydrogenation in the subsequent ammoxidation may lead to the formationof explosive gas mixtures. The by-products resulting from thedehydrogenation also restrict the onstream time of the ammoxidationcatalyst and lead to widening of the ammoxidation by-product spectrum.

It is an object of the present invention to provide an improved processfor preparing acrylonitrile from propane.

We have found that this object is achieved by a process for preparingunsaturated nitrites from the corresponding alkanes which comprises thesteps:

-   a) feeding an alkane into a dehydrogenation zone and catalytically    dehydrogenating the alkane to the corresponding alkene to obtain a    product gas stream A which comprises the alkene, unconverted alkane    and possibly one or more further gas components selected from the    group consisting of steam, hydrogen, carbon oxides, hydrocarbons    having a lower boiling point than the alkane or the alkene    (low-boilers), nitrogen and noble gases,-   b) at least partially removing further gas components from the    product gas stream A to give a feed gas stream B which comprises the    alkane and the alkene,-   c) feeding the feed gas stream B, ammonia, an oxygen-containing gas    and, if desired, steam into an oxidation zone and catalytically    ammoxidizing the alkene to the corresponding unsaturated nitrile to    give a product gas stream C which comprises the unsaturated nitrile,    ammoxidation by-products, unconverted alkane and alkene and possibly    one or more further gas components selected from the group    consisting of steam, oxygen, carbon oxides, ammonia, nitrogen and    noble gases,-   d) optionally removing ammonia from the product gas stream C to give    an ammonia-depleted product gas stream D,-   e) removing the unsaturated nitrile and ammoxidation by-products    from the product gas stream C or D by absorption in an aqueous    absorbent to give a gas stream E which comprises unconverted alkane    and alkene and possibly one or more further gas components selected    from the group consisting of oxygen, carbon oxides, ammonia,    nitrogen and noble gases, and an aqueous stream which comprises the    unsaturated nitrile and the by-products, and recovery of the    unsaturated nitrile from the aqueous stream,-   f) recycling the gas stream E into the dehydrogenation zone.

In a process step a), the alkane is fed into a dehydrogenation zone andcatalytically dehydrogenated to give the corresponding alkene.

Alkane starting materials for the process according to the invention aregenerally C₃-C₁₄-alkanes, and preference is given to propane andisobutane. The latter may be obtained, for example, from LPG (liquefiedpetroleum gas) or LNG (liquefied natural gas).

Alkanes may be dehydrogenated to give alkenes by oxidativedehydrogenation.

Oxidative alkane dehydrogenation may be carried out, for example, overMo/V mixed oxide catalysts as described in U.S. Pat. No. 4,250,346 orover NiO catalysts at temperatures of from 300 to 500° C. and alkaneconversions of from 10 to 20% as described in WO 00/48971.

Preference is given to carrying out alkane dehydrogenation as anon-oxidative catalytic dehydrogenation. This involves partiallydehydrogenating the alkane in a dehydrogenating reactor over adehydrogenating catalyst to give the alkene. The dehydrogenationresults, as well as hydrogen, in the formation of small quantities oflow-boiling hydrocarbon as cracking product of the alkane which, in thecase of propane dehydrogenation includes, for example, methane, ethaneand ethene. Depending on the dehydrogenation method, carbon oxides (CO,CO₂), water and nitrogen may also be present in product gas mixture A ofthe alkane dehydrogenation. In addition, unconverted alkane is presentin the product gas mixture.

The catalytic alkane dehydrogenation may be carried out with or withoutoxygen-containing gas as a co-feed.

Catalytic alkane dehydrogenation may in principle be carried out usingall reactor types and methods known from the prior art. A comparativelycomprehensive description of dehydrogenation processes used according tothe invention is also contained in “Catalytica® Studies Division,Oxidative Dehydrogenation and Alternative Dehydrogenation Processes”(Study Number 4192 OD, 1993, 430 Ferguson Drive, Mountain View, Calif.,94043-5272, USA).

A useful reactor type is the fixed bed tube reactor or tube bundlereactor. In these reactors, the catalyst (dehydrogenation catalyst and,where oxygen is used as co-feed, optionally a specialized oxidationcatalyst) is disposed as a fixed bed in a reaction tube or in a bundleof reaction tubes. The reaction tubes are customarily indirectly heatedby the combustion of a gas, for example a hydrocarbon such as methane,in the space surrounding the reaction tubes. It is favorable to applythis indirect form of heating only to about the first 20 to 30% of thelength of the fixed bed and to heat the remaining bed length to therequired reaction temperature by the radiant heat released in the courseof indirect heating. Customary reaction tube internal diameters are fromabout 10 to 15 cm. A typical dehydrogenation tube bundle reactorcomprises from about 300 to 1000 reaction tubes. The internaltemperature in the reaction tubes is customarily in the range from 300to 1200° C., preferably in the range from 600 to 1000° C. The workingpressure is customarily from 0.5 to 8 bar, frequently from 1 to 2 bar,when a small steam dilution is used (similar to the Linde process forpropane dehydrogenation), or else from 3 to 8 bar when a high steamdilution is used (similar to the steam active reforming process (STARprocess) for dehydrogenating propane or butane of Phillips PetroleumCo., see U.S. Pat. No. 4,902,849, U.S. Pat. No. 4,996,387 and U.S. Pat.No. 5,389,342). Typical gas hourly space velocities (GHSV) are from 500to 2000 h⁻¹, based on the alkane to be dehydrogenated. The catalystshape may, for example, be spherical or cylindrical (hollow or solid).

Catalytic alkane dehydrogenation, as described in Chem. Eng. Sci. 1992b, 47 (9-11) 2313, may also be carried out heterogeneously catalyzed inthe fluidized bed without diluting the alkane. It is advantageous tooperate two fluidized beds in parallel, of which one is generally in theprocess of regeneration. The operating pressure is typically from 1 to 2bar, the dehydrogenation temperature generally from 550 to 600° C. Theheat required for the dehydrogenation is introduced into the reactionsystem by preheating the dehydrogenation catalyst to the reactiontemperature. When an oxygen-containing co-feed is admixed, it ispossible to do without the preheater and to generate the required heatdirectly in the reactor system by combustion of hydrogen and/or ofhydrocarbons in the presence of oxygen. If desired, ahydrogen-containing co-feed may additionally be admixed.

Catalytic alkane dehydrogenation may be carried out in a tray reactor.This comprises one or more successive catalyst beds. The number ofcatalyst beds may be from 1 to 20, advantageously from 1 to 6,preferably from 1 to 4 and in particular from 1 to 3. The catalyst bedsare preferably flowed through radially or axially by reaction gas. Ingeneral, such a tray reactor is operated using a fixed catalyst bed. Inthe simplest case, the fixed catalyst beds are disposed axially in ashaft furnace reactor or in the annular gaps of concentric cylindricalgrids. A shaft furnace reactor corresponds to one tray. Carrying out thedehydrogenation in a single shaft furnace reactor corresponds to apreferred embodiment. In a further preferred embodiment, thedehydrogenation is carried out in a tray reactor having 3 catalyst beds.In a method which does not employ oxygen as a co-feed, the reaction gasmixture is subjected to intermediate heating in the tray reactor on itsway from one catalyst bed to the next catalyst bed, for example, bypassing it over heat exchanger plates heated by hot gases or by passingit through tubes heated by hot combustion gases.

In a preferred embodiment of the process according to the invention, thecatalytic alkane dehydrogenation is carried out autothermally. To thisend, the reaction gas mixture of the alkane dehydrogenation isadditionally admixed with oxygen in at least one reaction zone and thehydrogen contained in the reaction gas mixture is combusted, whichdirectly generates in the reaction gas mixture at least a portion of theheat required for dehydrogenation in the at least one reaction zone. Acharacteristic of the autothermal method compared to what could betermed an oxidative method is, for example, the presence of hydrogen inthe effluent gas. The oxidative processes form no significant quantitiesof free hydrogen.

In general, the quantity of the oxygen-containing gas added to thereaction gas mixture is chosen in such a way that the heat quantityrequired for the dehydrogenation of the alkane is generated by thecombustion of the hydrogen present in the reaction gas mixture and/or ofthe hydrocarbon present in the reaction gas mixture and/or of thehydrocarbons present in the form of coke. In general, the overallquantity of oxygen added, based on the total quantity of the alkane, isfrom 0.001 to 0.5 mol/mol, preferably from 0.005 to 0.2 mol/mol, morepreferably from 0.05 to 0.2 mol/mol. Oxygen may either be added as pureoxygen or else as an oxygen-containing gas in a mixture with inertgases. Preference is given to using air as the oxygen-containing gas.The inert gases and the gases resulting from combustion generallyprovide additional dilution and therefore support the heterogeneouslycatalyzed dehydrogenation.

The hydrogen combusted to generate heat is the hydrogen formed bycatalytic alkane dehydrogenation and also any hydrogen additionallyadded to the reaction gas mixture. The quantity of hydrogen shouldpreferably be such that the H₂/O₂ molar ratio in the reaction gasmixture immediately after the oxygen is fed in is from 2 to 10 mol/mol.In multistage reactors, this applies to every intermediate oxygenfeedpoint and any intermediate hydrogen feedpoint.

The hydrogen is combusted catalytically. The dehydrogenation catalystused generally also catalyzes the combustion of the hydrocarbons and ofhydrogen with oxygen so that in principle no additional specializedoxidation catalyst is necessary. In one embodiment, operation iseffected in the presence of one or more oxidation catalysts whichselectively catalyze the combustion of hydrogen to oxygen in thepresence of hydrocarbons. The combustion of hydrocarbons with oxygen togive CO and CO₂ only occurs to a minor extent, which has a distinctpositive effect on the selectivities achieved for the alkene formation.The dehydrogenation catalyst and the oxidation catalyst are preferablypresent in different reaction zones.

When the reaction is carried out in more than one step, the oxidationcatalyst may be present in only one, in more than one or in all thereaction zones.

Preference is given to disposing the catalyst which selectivelycatalyzes the oxidation of hydrogen at the locations where there arehigher oxygen partial pressures than at other locations in the reactor,in particular near the feedpoint for the oxygen-containing gas. Theoxygen-containing gas and/or hydrogen may be fed in at one or morelocations in the reactor.

In one embodiment of the process according to the invention,intermediate feeding in of oxygen-containing gas and of hydrogen occursbefore every tray of a tray reactor. In a further embodiment of theprocess according to the invention, metering in of oxygen-containing gasand of hydrogen occurs before every tray except the first tray. In oneembodiment, a layer of a specialized oxidation catalyst is presentdownstream of every feedpoint, followed by a layer of dehydrogenationcatalyst. In a further embodiment, no specialized oxidation catalyst ispresent. The dehydrogenation temperature is generally from 400 to 1100°C., the pressure in the last catalyst bed of the tray reactor generallyfrom 0.2 to 5 bar, preferably from 1 to 3 bar. The GHSV is generallyfrom 500 to 2000 h⁻¹, and in high-load operation even up to 100 000 h⁻¹,preferably from 4000 to 16 000 h⁻¹, based on the alkane to bedehydrogenated.

A preferred catalyst which selectively catalyzes the combustion ofhydrogen comprises oxides or phosphates selected from the groupconsisting of oxides or phosphates of germanium, tin, lead, arsenic,antimony, indium and bismuth. A further preferred catalyst whichcatalyzes the combustion of hydrogen comprises a noble metal oftransition group VIII or I of the Periodic Table.

The dehydrogenation catalysts used generally have a support and anactive composition. The support consists of a heat-resistant oxide ormixed oxide. The dehydrogenation catalysts preferably comprise a metaloxide selected from the group consisting of zirconium dioxide, zincoxide, aluminum oxide, silicon dioxide, titanium dioxide, magnesiumoxide, lanthanum oxide, cerium oxide and mixtures thereof, as support.The mixtures may be physical mixtures or else chemical mixed phases suchas magnesium aluminum oxide or zinc aluminum oxide mixed structures.Preferred supports include zirconium dioxide and/or silicon dioxide, andparticular preference is given to mixtures of zirconium dioxide andsilicon dioxide.

The active composition of the dehydrogenation catalysts generallycomprises one or more elements of transition group VIII of the PeriodicTable, preferably platinum and/or palladium, more preferably platinum.The dehydrogenation catalysts may further comprise one or more elementsof main group I and/or II of the Periodic Table, preferably potassiumand/or cesium. The dehydrogenation catalysts may further contain one ormore elements of transition group III of the Periodic Table includingthe lanthanides and actinides, preferably lanthanum and/or cerium.Finally, the dehydrogenation catalysts may have one or more elements ofmain group III and/or IV of the Periodic Table, preferably one or moreelements selected from the group consisting of boron, gallium, silicon,germanium, tin and lead, more preferably tin.

In a preferred embodiment, the dehydrogenation catalyst comprises atleast one element of transition group VIII, at least one element of maingroup I and/or II, at least one element of main group III and/or IV andat least one element of transition group m including the lanthanides andactinides, of the Periodic Table.

For example, all dehydrogenation catalysts which are disclosed by WO99/46039, U.S. Pat. No. 4,788,371, EP-A 705,136, WO 99/29420, U.S. Pat.No. 5,220,091, U.S. Pat. No. 5,430,220, U.S. Pat. No. 5,877,369, EP 0117 146, DE-A 199 37 106, DE-A 199 37 105 and DE-A 199 37 107 may beused according to the invention. Particularly preferred catalysts forthe above-described variants of autothermal alkane dehydrogenationinclude the catalysts according to Examples 1, 2, 3 and 4 of DE-A 199 37107.

Preference is given to carrying out the alkane dehydrogenation in thepresence of steam. The steam added serves as a heat carrier and supportsthe gasification of organic deposits on catalysts, which counteractscarbonization of the catalysts and increases the onstream time of thecatalyst. The organic deposits are converted to carbon monoxide andcarbon dioxide.

The dehydrogenation catalyst may be regenerated by methods known per se.For instance, steam can be added to the reaction gas mixture or, fromtime to time, an oxygen-containing gas may be passed over the catalystbed at elevated temperature and the deposited carbon burnt off. Thedehydrogenation catalyst may then be reduced in a hydrogen-containingatmosphere.

The alkane dehydrogenation may also be carried out by the circuit gasmethod described in the yet to be published German patent application P102 11 275.4.

The alkane dehydrogenation gives a product gas mixture A which, as wellas the alkene and unconverted alkane, comprises secondary components.Customary secondary components include hydrogen, water, nitrogen, CO andCO₂ and also hydrocarbons which have lower boiling points than thealkane and the alkene (low-boilers) which, in the case of propanedehydrogenation, include, for example, methane, ethane and ethene ascracking products. In the case of isobutane dehydrogenation, propane,propene, propine and allene may also be present as cracking products.The composition of the gas mixture leaving the dehydrogenation stage maybe highly variable depending on the dehydrogenation method. Forinstance, when the preferred autothermal dehydrogenation with feeding inof oxygen and additional hydrogen is carried out, the product gasmixture A will have a comparatively high content of water and carbonoxides. Methods without feeding in of oxygen will provide adehydrogenation product gas mixture A having a comparatively highhydrogen content. Customarily, it will be under a pressure of from 0.3to 10 bar and frequently a temperature of 400 to 1200° C., in favorablecases from 450 to 800° C.

In a process step b), the further gas components other than the alkaneand the alkene are at least partially, but preferably almost completely,removed from the product gas stream A.

The product gas stream leaving the dehydrogenation zone may be separatedinto two substreams, and only one of the two substreams may be subjectedto the further process steps b) to f) as product gas stream A, while thesecond substream may be recycled directly into the dehydrogenation zone.However, preference is given to subjecting the entire dehydrogenationproduct gas stream to the further process steps b) to f) as product gasstream A.

In one embodiment of the process according to the invention, water isremoved first. The water may be removed, for example, by condensing outby cooling and/or compressing the dehydrogenation product gas stream Aand may be carried out in one or more cooling and/or compressing stages.Water removal is customarily carried out when the alkane dehydrogenationis carried out autothermally or is carried out isothermally with feedingin of steam (similarly to the Linde or STAR process for dehydrogenatingpropane) and consequently the product gas stream has a high watercontent.

The removal of the further gas components other than the alkane andalkene from the product gas stream may be carried out by customaryseparation processes such as distillation, rectification, membraneprocesses, absorption or adsorption.

The removal of the hydrogen contained in the product gas mixture A fromthe alkane dehydrogenation may be effected, optionally after cooling,for example in an indirect heat exchanger, may be passed through amembrane, generally in the form of a pipe, which is only permeabletoward molecular hydrogen. If required, the removed molecular hydrogenmay be at least partially used in the dehydrogenation or else beutilized in a different way, for example for generating electricalenergy in fuel cells.

The carbon dioxide contained in the dehydrogenation product gas stream Amay be removed by CO₂ gas scrubbing. The carbon dioxide gas scrubbingmay precede a separate combustion stage in which carbon monoxide isselectively oxidized to give carbon dioxide.

In a preferred embodiment of the process according to the invention, thealkane and the alkene are removed from the noncondensable or low-boilinggas components such as hydrogen, carbon oxides, low-boiling hydrocarbonsand, if present, nitrogen in an absorption/desorption cycle by means ofa high-boiling absorbent to give a reaction gas stream b whichessentially consists of the alkane and the alkene.

To this end, the product gas stream A, optionally after preceding waterremoval, is brought into contact with an inert absorbent in anabsorption step and the alkane and the alkene are absorbed by the inertabsorbent to give an absorbent loaded with the alkane and alkene and anoffgas comprising the remaining gas components. In a desorption step,the alkane and the alkene are released from the absorbent.

Inert absorbents used in the absorption stage are generally high-boilingnonpolar solvents in which the alkane/alkene mixture to be removed has adistinctly higher solubility than the remaining components of theproduct gas mixture. The absorption may be effected by simply passingthe dehydrogenation product gas mixture through the absorbent. It mayalso be effected in columns or in rotation absorbers. Operation may beeffected in cocurrent, countercurrent or cross current. Examples ofuseful absorption columns include tray columns having bubble cap trays,centrifugal trays and/or sieve trays, columns having structuredpackings, for example, sheet metal packings having a specific surfacearea of from 100 to 1000 m²/m³ such as Mellapak® 250 Y, and columnshaving random packings. It is also possible to use trickle towers andspray towers, graphite block absorbers, surface absorbers such as thickfilm and thin film absorbers and rotary columns, plate scrubbers,cross-spray scrubbers and rotary scrubbers.

Useful absorbents include comparatively nonpolar organic solvents, forexample aliphatic C₈- to C₁₈-alkenes, or aromatic hydrocarbons such asthe middle oil fractions from paraffin distillation, or ethers havingbulky groups, or mixtures of these solvents, and a polar solvent such as1,2-dimethyl phthalate. Further useful absorbents include esters ofbenzoic acid and phthalic acid with straight-chain C₁-C₈-alkanols, suchas n-butyl benzoate, methyl benzoate, ethyl benzoate, dimethylphthalate, diethyl phthalate, and also what are known as heat carrieroils, such as biphenyl and diphenyl ether, chloro derivatives thereofand also triarylalkenes. A useful absorbent is a mixture of biphenyl anddiphenyl ether, preferably in the azeotropic composition, for examplethe commercially obtainable Diphyl®. This solvent mixture frequentlycomprises dimethyl phthalate in a quantity of from 0.1 to 25% by weight.Useful absorbents also include octanes, nonanes, decanes, undecanes,dodecanes, tridecanes, tetradecanes, pentadecanes, hexadecanes,heptadecanes and octadecanes and fractions obtained from refinerystreams which have the linear alkanes mentioned as the main components.

Desorption is carried out by heating the loaded absorbent and/ordepressurizing it to a lower pressure. Alternatively, desorption mayalso be carried out by stripping or by a combination of depressurizing,heating and stripping in one or more process steps. The absorbentregenerated in the desorption stage is recycled into the absorptionstage.

A feed gas stream B is obtained which comprises the alkane and thealkene and is substantially free of the further gas components.

In one process variant, the desorption step is carried out bydepressurizing and/or heating the loaded absorbent. In this case, a feedgas stream B is obtained which essentially consists of the alkane andthe alkene.

When the desorption is carried out according to a further processvariant by (additional) stripping of the absorbent, the feed gas streamB comprises the stripping gas, as well as the alkane and the alkene. Inan advantageous process variant, the stripping gas used is anoxygen-containing gas in the quantities required for the subsequentammoxidation.

The removal b) is generally incomplete so that, depending on the methodof removal, small quantities or else only traces of the further gascomponents (for example the low-boiling hydrocarbons) may still bepresent in the feed gas stream B.

The at least partial removal of the further gas components from thedehydrogenation product gas stream A before feeding the gas stream intothe ammoxidation brings a series of advantages. For instance, theformation of explosive gas mixtures in the ammoxidation reactor bypreceding removal of hydrogen formed by the dehydrogenation is avoided.The removal of the by-products resulting from the dehydrogenation, forexample the low-boiling cracking products of the alkanes to bedehydrogenated, firstly has a positive effect in the subsequentcatalytic ammoxidation on the stability of the catalyst, whose onstreamtime is increased. Secondly, by-product formation, for example theformation of acetaldehyde and acetic acid from ethylene, is suppressed.

In a process step c), the feed gas stream B comprising the alkane andthe alkene, ammonia, oxygen-containing gas and, if desired, steam arefed into an oxidation zone and an ammoxidation of the alkene to thecorresponding unsaturated nitrile is carried out.

The catalytic ammoxidation is carried out in a manner known per se. Theammoxidation is customarily carried out at temperatures of from 375 to550° C. and pressures of from 0.1 to 10 bar at a molar ratio of ammoniato alkene of from 0.2:1 to 2:1. Useful catalysts are known to thoseskilled in the art and described, for example, in WO95/05241, EP-A 0 573713, U.S. Pat. No. 5,258,543 and U.S. Pat. No. 5,212,317. Theammoxidation may be carried out in a tube reactor which contains thecatalyst in particulate form and is surrounded by a cooling liquid fordissipating the heat of reaction. Preference is given to carrying outthe ammoxidation in a fluidized bed reactor. The volume ratio of oxygento alkene is customarily from 1.6:1 to 2.4:1. The volume ratio ofammonia to alkene is customarily from 0.7:1 to 1.2:1.

The oxygen-containing gas which is fed into the oxidation zone may bepure oxygen, air or oxygen-enriched or oxygen-depleted air. Thepreferred oxygen-containing gas is air.

A product gas stream C is obtained which comprises the unsaturatednitrile, ammoxidation by-products and unconverted alkane and alkene, andmay possibly comprise steam, oxygen, carbon oxides, ammonia, nitrogenand/or noble gases.

For example, the product gas stream C of ammoxidation of propene to giveacrylonitrile may comprise ammoxidation by-products acrolein,acetonitrile and HCN. The product gas stream C of ammoxidation ofisobutene to give methacrylonitrile may comprise ammoxidationby-products methacrolein, HCN, acetonitrile and acrylonitrile.

In general, but not necessarily, the ammoxidation product gas stream Calso comprises oxygen, ammonia and frequently also carbon oxides. Whenoperation is effected using air as the oxygen-containing gas, itcomprises nitrogen and noble gases and, when operation is effected whilefeeding in steam, also comprises steam.

Optionally, ammonia may be removed from the product gas stream C in aprocess step d) to give a highly ammonia-depleted or an ammonia-freeproduct gas stream D.

In one process variant, a separate ammonia removal d) is effected bybringing the hot ammoxidation product gas stream C into contact withaqueous sulfuric acid in a quenching tower and thus washing ammonia outof the product gas stream C as ammonium sulfate. This gives an aqueousammonium sulfate solution which may comprise dissolved unsaturatednitrile and also ammoxidation by-products. These may be stripped out ofthe aqueous ammonium sulfate solution in a downstream vapor stripperusing steam and fed to further distillative workup.

In a further process variant, no separate ammonia removal d) iseffected. However, ammonia is substantially, if not completely, removedfrom the ammoxidation product gas stream in the subsequent absorptionstep e) by absorption in the aqueous absorbent.

Alternatively, ammonia may also be removed from the ammoxidation productgas mixture by feeding methanol, which reacts with ammonia to give HCN,water and carbon dioxide, into the upper portion of the fluidized bedreactor where the ammoxidation is carried out (from about 85 to 95% ofthe total length).

In one process step e), the unsaturated nitrile and any ammoxidationby-products are removed from the product gas stream C or D by absorptionin an aqueous absorbent. To this end, the product gas stream C or D isbrought into contact with the aqueous absorbent in a gas scrubber togive an aqueous stream comprising the unsaturated nitrile, anyammoxidation by-products and any ammonia, from which the unsaturatednitrile is subsequently recovered, and a gas stream E which comprisesunconverted alkane and alkene and any oxygen, carbon oxides, ammonia,nitrogen and/or noble gases.

When the product gas stream from which the unsaturated nitrile is washedout by the aqueous absorbent still comprises significant ammoniaquantities, because, for example, there was no ammonia removal d),ammonia will be at least partially dissolved in the aqueous absorbent bythe formation of ammonium carbonate in the presence of carbon dioxidelikewise present in the product gas stream.

The unsaturated nitrile is recovered from the aqueous stream obtained inthe absorption stage by distillation. For example, in the case ofacrylonitrile preparation from propane, the aqueous stream resultingfrom the absorption stage may be separated in a first distillationcolumn into a top stream consisting of crude acrylonitrile and a bottomstream comprising acetonitrile, water and high-boilers. The crudeacrylonitrile obtained as the top stream, which in particular alsocontains HCN, may be further purified by distillation. Pure acetonitrilemay be recovered from the bottom stream by distillation. The workup inthe case of methacrylonitrile preparation is similar.

Finally, the gas stream E, which comprises unconverted alkane and alkeneand may comprise oxygen, carbon oxides, ammonia, nitrogen and/or noblegases, is recycled into the dehydrogenation zone (step a)). The presenceof ammonia in the recycled gas stream E is not disadvantageous for thealkane dehydrogenation. This is oxidized in the autothermaldehydrogenation method to give nitrogen or nitrogen oxides.

The presence of (residual) oxygen in the recycled gas stream E reducesthe thermodynamic limitation of the alkane dehydrogenation, since theresidual oxygen reacts with the hydrogen formed in the dehydrogenationand the equilibrium is thus shifted toward the product side. Since thedehydrogenation results in the volume increase, dilution by gases in thegas stream E likewise has a positive effect on the equilibrium point.

1. A process for preparing unsaturated nitriles from the correspondingalkanes which comprises the steps: a) feeding an alkane and oxygen or anoxygen-containing gas into a dehydrogenation zone and catalyticallydehydrogenating the alkane to the corresponding alkene to obtain aproduct gas stream A which comprises the alkene, unconverted alkane andone or more further gas components selected from the group consisting ofsteam, hydrogen, carbon oxides, hydrocarbons having a lower boilingpoint than the alkane or the alkene (low-boilers), nitrogen and noblegases, wherein the dehydrogenation is carried out autothermally, b) atleast partially removing further gas components from the product gasstream A to give a feed gas stream B which consists essentially of thealkane and the alkene, by a process comprising contacting the productgas stream A with an inert absorbent in an absorption stage wherein thealkane and the alkene are absorbed by the inert absorbent to give anabsorbent laden with the alkane and alkene and an offgas streamcomprising the remaining gas components, and releasing the alkane andthe alkene from the absorbent in a desorption stage, c) feeding the feedgas stream B, ammonia, an oxygen-containing gas and, if desired, steaminto an oxidation zone and catalytically ammoxidizing the alkene to thecorresponding unsaturated nitrile to give a product gas stream C whichcomprises the unsaturated nitrile, ammoxidation by-products, unconvertedalkane and alkene and possibly one or more further gas componentsselected from the group consisting of steam, oxygen, carbon oxides,ammonia, nitrogen and noble gases, d) optionally removing ammonia fromthe product gas stream C to give an ammonia-depleted product gas streamD, e) removing the unsaturated nitrile and ammoxidation by-products fromthe product gas stream C or D by absorption in an aqueous absorbent togive a gas stream E which comprises unconverted alkane and alkene andpossibly one or more further gas components selected from the groupconsisting of oxygen, carbon oxides, ammonia, nitrogen and noble gases,and an aqueous stream which comprises the nitrile and the by-products,and recovery of the unsaturated nitrile from the aqueous stream, f)recycling the gas stream E directly into the dehydrogenation zone.
 2. Aprocess as claimed in claim 1, wherein the absorption stage is precededby a water removal operation.
 3. A process as claimed in claim 1,wherein, in step d), ammonia is removed as ammonium sulfate by scrubbingwith sulfuric acid.
 4. A process as claimed in claim 1, wherein thealkane is propane, the alkene is propene and the unsaturated nitrile isacrylonitrile.
 5. A process as claimed in claim 1, wherein the alkane isisobutane, the alkene is isobutene and the unsaturated nitrile ismethacrylonitrile.